Bismaleimide resins are used to saturate fibrous reinforcements to form heat curable prepregs. When these are consolidated under heat and pressure, laminated shapes, useful as high strength, low weight airfoils are obtained. The prepregs however have some disadvantages, such as poor tack and drape. This has been overcome in one way by using a reactive divinyl aryl diluent for the resin, such as divinylbenzene, see, U.S. Pat. No. 4,351,932. It has also been proposed to use several divinyl nitrogen containing families of compounds for the same purpose, and these lead to laminates with unexpected additional advantages, such as increased fire resistance and increased toughness. A particularly useful group of such compounds are vinylstilbazoles derived from methyl pyridines and aromatic monoaldehydes, and they can be represented by the following general formula: ##STR1## wherein R.sup.1 is hydrogen, nitro, halogen, e.g., chloro, trifluoromethyl, and the like. The vinylstilbazoles have other uses, but they are particularly adapted to act as cross-linking agents for ethylenically unsaturated bis-imides. An improved process to prepare them is the subject matter of applicant's commonly assigned copending application Ser. No. 575,096, filed Jan. 30, 1984.
Another family of divinyl nitrogen compounds are the vinylpolystyrylpyridines derived from polymethylpyridines, aromatic dialdehydes and vinylmethylpyridine. These are described in U.S. Pat. No. 4,362,860 and
have the typical formula EQU E--AP.sub.n-1 AE
wherein P is derived from the methylated pyridine, A is derived from the aromatic dialdehyde, E is derived from the methylvinylpyridine and n is the number of repeating units in the chain. Although the '860 patent describes the application of such compounds to a fiber or fabric to make the prepregs, a primary utility is, as has been mentioned above, in cross-linking bis maleimides in prepreg composites.
The process employed in the '860 patent to make vinylpolystyrylpyridines comprises mixing all of the three ingredients under aldol condensation conditions. This makes it difficult to control the molecular weight of the end product because of cross-linking and rapid reaction rates. Final product distribution is unpredictable and when such products are used as cross-linking agents, laminates with differing properties are ultimately obtained.
It has now been discovered that such disadvantages can be avoided and superior products are produced if the reaction is carried out in two steps: First, a polystyrylpyridine oligomer adduct (I) is generated through the reaction of a polymethylpyridine and an aromatic dialdehyde: ##STR2## wherein R.sup.1 is hydrogen, or an electron withdrawing group, such as nitro, methoxy, chloro, trifluoromethyl, and the like and n is 0 or a whole number of from 1 to about 6, usually about 2. Second, a methylvinylpyridine compound is then added to react with the terminal aldehyde groups to yield the desired product (II): ##STR3##
Because of the possibility of cross-linking via a third methyl group, for example if 2,4,6 trimethylpyridine is used, and because of the rapid reaction between such compounds and the aromatic dialdehydes, the timing of the addition of the methylvinylpyridine is crucial in determining the molecular weight of the final product. At least two ways of determining this are possible, one is to withdraw an aliquot of the first stage reaction mixture, cool it and determine whether or not a precipitate forms. The methylvinylpyridine should not be added until a precipitate is first noticed. This is usually 2 to 3 hours into the run. A preferred method is based on the appearance of an adduct by gas chromatographic analysis. When the ratio of this adduct, e.g., of 2,4,6-trimethylpyridine and terephthaldehyde is 4.3-4.8 by gas chromatography, the methylvinylpyridine should be added. Either method will provide reproducible molecular weights and ultimately a superior and novel product.